1. Field of the Invention
The invention relates to a voltage controlled oscillator, and more particularly to a voltage controlled oscillator and voltage controlled oscillator applied in a phase-frequency locked loop circuit.
2. Description of the Related Art
A conventional voltage control ring oscillator is formed by n-stages delay cells coupled serially in a ring loop. All the delay cells are controlled by the same adjustable control voltage through their voltage control input terminals, and the oscillation frequency of the voltage controlled ring oscillator is thus obtained. FIG. 1 shows a voltage-to-frequency transfer curve of a conventional voltage control ring oscillator. The slope of the voltage-to-frequency transfer curve is calculated to serve as the gain of the voltage control ring oscillator, as the equation E1:Kvco=dF/dVC  (E1)
wherein Kvco represents the gain of the voltage control ring oscillator, dVC represents a control voltage difference, and dF represents a frequency difference corresponding to the control voltage difference.
When a voltage control ring oscillator is applied in a phase locked loop (PLL) circuit, a smaller Kvco is better for weak phase jitter. Thus, a voltage control ring oscillator with smaller Kvco is desired for a phase locked loop circuit.
With greater requirements from high-speed electronic devices, the center frequency (Fc) of a voltage control ring oscillator is increased by decreasing the number (n) of the stages of delay cells, decreasing node capacitance (Cp) of the voltage control ring oscillator, or increasing current (Iss) of each delay cell. FIG. 2 shows transfer curves of a 4-stage voltage control ring oscillator in different conditions. Referring to FIG. 2, from curve A to curve C, as the arrow 10 indicates, the center frequency (Fc) increases when n is decreased, Cp is decreased, and Iss is increased. However, the consumption current is increased, and Kvco also increases, undesirably. As shown In FIG. 2, curve A with the lowest center frequency Fc_L has the smallest Kvco, curve C with the highest center frequency Fc_H has the highest Kvco, and curve B with the medium enter frequency Fc_M has a medium Kvco.
Thus, for high-speed devices, a voltage control ring oscillator with a high center frequency and a small gain (Kvco) is desirable.